Dogs which will develop progressive retinal atrophy will be studied using electroretinography to determine the normal functional development of the rod and cone photoreceptor systems. In addition, these dogs will be used to investigate the temporal relation between the onset of rod and cone dysfunction. A second group of dogs affected with the same disease will be studied both electrophysiologically and ultrastructurally before, during, and after the onset of cone degeneration. This study will determine if the rod and cone degenerative processes occur concurrently and the structural basis for the functional deficit. Autoradiographic studies will determine the role of photoreceptor renewal on the mechanism of disease.